I have FF 40.0.2 as well and Kentico 8.0.21, 8.1.17 and 8.2.34 are working fine.
Have you tried clearing the browser cache?
Also, there was a bug in Kentico 8.2 related to slow connection - in some cases the dashboard was not loaded correctly:
Application dashboard - The application dashboard didn't always load correctly for slow connections
The application dashboard did not always load correctly the first time it was requested. This happened to users with slow connections to the server.
It was fixed in hotfix 8.2.30